Background: Liver fibrosis is an important clinical endpoint of progression of autoimmune liver disease (AILD); its monitoring would benefit from noninvasive imaging tools. Objective: To assess the relationship between MR elastography (MRE) liver stiffness measurements and histologic liver fibrosis, as well as to evaluate the performance of MRE and biochemical-based clinical markers for stratifying histologic liver fibrosis severity, in children and young adults with AILD. Methods: This retrospective study used an existing institutional registry of children and young adults diagnosed with AILD [primary sclerosing cholangitis (PSC), ASC (autoimmune sclerosing cholangitis), or AIH (autoimmune hepatitis)]. The registry was searched to identify patients who underwent both a research abdominal 1.5-T MRI that included liver MRE (performed for registry enrollment) and a clinically indicated liver biopsy within a 6-month interval. MRE used a 2D gradient-recalled echo sequence. One analyst measured mean liver shear stiffness (kPa) for each examination. Laboratory markers of liver fibrosis [aspartate aminotransferase-to-platelet ratio index (APRI) and fibrosis-4 score (FIB-4)] were recorded. For investigational purposes, one pathologist determined histologic METAVIR liver fibrosis stage, blinded to clinical and MRI data. Spearman rank-order correlation was calculated between MRE liver stiffness and METAVIR liver fibrosis stage. ROC analysis was used to evaluate diagnostic performance for identifying advanced fibrosis (i.e., differentiating METAVIR F0-F1 from F2-F4 fibrosis), calculating sensitivity and specificity at the Youden's index. Results: The study included 46 patients (median [IQR] age, 16.6 [13.7-17.8] years; 20 female, 26 male); 12 had PSC, 10 had ASC, and 24 had AIH. Median MRE liver stiffness was 2.9 kPa (IQR: 2.2-4.0 kPa). MRE liver stiffness and METAVIR fibrosis stage showed strong positive correlation (rho=0.68). For identifying advanced liver fibrosis, MRE liver stiffness had AUC of 0.81, with sensitivity of 65.4% and specificity of 90.0%; APRI had AUC of 0.72, with sensitivity of 60.0% and specificity of 85.0%; and FIB-4 had AUC of 0.71, with sensitivity of 64.0% and 85.0%. Conclusion: MRE liver stiffness measurements were associated with histologic liver fibrosis severity. Clinical Impact: The findings support a role for MRE in noninvasive monitoring of liver stiffness, a surrogate for fibrosis, in children and young adults with AILD.